1. Field of the Invention
The present invention relates to a digital television (TV) broadcast signal receiver that can receive not only digital television broadcast signals such as ATSC (Advanced television systems Committee), but also analogue television broadcast signals such as NTSC National Television System Committee).
2. Description of the Related Art
In the North American Continent where an ATSC digital television broadcast (it is abbreviated to a hereinafter digital TV broadcast) is performed, plains are dotted with metropolises, so that various TV broadcast signals broadcasted in the suburbs of each metropolis can be received at other cities or the halfway point of the cities. In consideration from a user who receives the TV broadcast signals, since many TV broadcast signals are transmitted in various directions, it is necessary to regulate a direction of an antenna in an orientation to a TV broadcasting station from which a program which he wants to watch is delivered. Therefore, a multi-directional antenna such as a smart antenna is put into practical use.
With respect to an apparatus for controlling a signal receiving direction of an antenna, Publication of Japanese Patent Application No. 5-232205 shows an apparatus mounted on a moving body such as a vehicle for following a communication satellite. Publication of Japanese Patent Application No. 2003-60423 shows a smart antenna used for communicating between a mobile phone and a base station.
According to the EIA-909 technical standard, it is provided that a smart antenna has sixteen signal receiving directions in respective directions where a circle is divided into sixteen. A digital TV broadcast signal receiver in compliance with the EIA-909 technical standard can switch an effective signal receiving direction of the multi-directional antenna such as the smart antenna to one of all the sixteen signal receiving directions. In addition, the digital TV broadcast signal receiver can receive conventional NTSC analogue TV broadcast signals.
There are digital TV broadcast signal receivers having an automatic scanning function for selecting a signal receiving direction of the multi-directional antenna in which a TV broadcast signal of a channel chosen by a user can be received at the best condition. In general, an antenna of a TV station, that is, a position of a TV tower is invariance. Thus, a TV broadcast signal of each channel is searched in each sixteen signal receiving direction as an initial processing of the digital TV broadcast signal receiver, and a signal receiving direction, in which the signal receiving condition is the most suitable, is set as an optimum signal receiving direction of each channel. For example, in the United States, a TV station can be established relatively easier. In contrast, a TV station suddenly becomes bankrupt. Therefore, it happens that a new channel, which cannot be received beforetime, can be received, or an old channel receivable long before cannot be received. Furthermore, a signal receiving condition of a TV broadcast signal of a channel is varied due to construction of a high-rise building. Thus, it becomes necessary to reset an optimum signal receiving direction for receiving a TV broadcast signal of a channel chosen by a user.
In general, a direction showing the largest value of the sensitivity of the TV broadcast signal among the directions in which the TV broadcast signal can be received is defined as an optimum signal receiving direction with respect to each channel. However, when a megastructure such as the high-rise building exists in the vicinity of a multi-directional antenna, a reflected TV broadcast signal reflected by the megastructure is superimposed on a TV broadcast signal directly transmitted from a TV station of a predetermined channel in a signal receiving direction of the multi-directional antenna, so that it is possible that a sensitivity of the TV broadcast signal in the signal receiving direction becomes larger than a sensitivity of a TV broadcast signal directly transmitted from the TV station. In such a case, if the direction showing the largest value of the sensitivity of the TV broadcast signal is regarded as an optimum signal receiving direction of the channel, color signals and/or horizontal synchronized signal cannot be received normally due to ghost image, since the reflected TV broadcast signal is superimposed an the TV broadcast signals directly transmitted from the TV station. Consequently, the colors of the image cannot be reproduced on a monitor screen, or image displayed on the monitor screen rolls from side to side. By the way, when the TV broadcast signal is digital signal, the digital TV broadcast signal receiver has error correction function, so that it is possible to judge the optimum signal receiving direction while error correction process. Accordingly, the above-mentioned problem relates to analogue TV broadcast signals, essentially.